The invention relates to a biosensor for measuring changes in viscosity or density in a test fluid, such as the coagulation of blood. The biosensor includes a housing surrounding a measuring chamber in which a piezoelectric element, and in particular a quartz crystal operating in a shear mode of vibration, is disposed. The piezoelectric element constitutes an oscillating unit with a measuring surface adapted to be wetted with a measuring mixture comprising the test fluid and a reaction component. Changes in the parameters of the oscillating are evaluated by a suitable electronic evaluation circuit.
European Patent EP-B 177 858, discloses a biosensor arrangement with which the coagulation rate of blood can be measured. A quartz crystal which is connected in the capacity of a resonant circuit to an oscillator having a fixed frequency is disposed in a measuring chamber which can be closed with a lid. The test fluid, which comprises the blood to be examined, is initially mixed with a coagulation component and applied to the measuring surface of the quartz crystal, upon which the coagulation rate of the blood is measured by evaluating a decrease in the vibration amplitude of the quartz crystal caused by a damping or mistuning of the resonant circuit by the coagulating blood. The elapsed time from the time the blood is mixed with the coagulation component up to the time of coagulation is measured by an electronic stopwatch. One disadvantage of the above arrangement is that, after a measurement has been made, the measuring chamber of the biosensor or the measuring surface of the quartz crystal can be cleaned only with great difficulty. Another disadvantage of this arrangement is that the elapsed time from the time the blood is mixed with the coagulation component until its application to the measuring surface must be carefully taken into account and monitored to avoid incorrect measurements. Yet another disadvantage of the above arrangement is the necessity for the relatively complicated procedure of initially mixing the blood with the coagulation component before applying it to the piezoelectric element. Similar devices are also disclosed in Japanese Patent Publications JP-A 62/153761 and JP-A 40/32767.
Biosensors for detecting antigen-antibody reactions which work with piezoelectric elements, for instance with a quartz crystal are also known. In such biosensors, an antigen-antibody is applied to the measuring surface of the piezoelectric element, upon which the piezoelectric element is dipped into the test fluid in order to measure and evaluate the resultant mistuning of the piezoelectric element as an oscillating circuit. Biosensors of this kind are described in U.S. Pat. Nos. 4,236,893 and 4,735,906. These biosensors have the disadvantage that the selectively absorbent layer of the oscillator surface must be cleaned before each new use, which procedure is very tedious and can gradually destroy the absorbent layer. Moreover, blood coagulation sensors of the above type in which the reaction component is initially applied to the measuring surface of the piezoelectric element would have the disadvantage that the piezoelectric element would be pre-strained by the reaction component, such that it would no longer be possible to make a measurement of the oscillating frequency of the piezoelectric element in the unstrained state.